Cement-directing orthopedic implants

ABSTRACT

A cement-directing structure for use in cement-injection bone therapy includes a collapsible, self-restoring braided structure with regions of differential permeability to the bone cement. The regions of differential permeability may be provided by areas where the braided mesh density is greater or lesser than surrounding areas and/or by means of a baffle. After the structure is placed in a void within a bony structure, cement is injected into the interior of the structure then oozes out in preferred directions according to the locations of the regions of differential permeability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority to U.S. patentapplication Ser. No. 13/331,676 filed on Dec. 20, 2011 which is based onand claims priority to provisional U.S. patent application Ser. No.60/562,686 filed Apr. 15, 2004 and provisional U.S. patent applicationSer. No. 60/604,800 filed Aug. 26, 2004, the contents of both of whichare incorporated by reference.

FIELD OF THE INVENTION

In general, the invention relates to orthopedic implants. Moreparticularly, the invention relates to devices that are used tofacilitate bone cement treatment of vertebral or other bone defects.

BACKGROUND OF THE INVENTION

There are many disease states and abnormal conditions that cause defectsin the skeleton. For instance, osteoporosis and other metabolic boneconditions weaken the bone structure and predispose the bone tofracture. If not treated, certain fractures and bone defects mayprogress and lead to the development of severe neurological or othermedical complications.

Other examples of bone defects are those resulting from the excision ofbenign or malignant lesions of the skeleton. Proliferation of tumorsoften compromises the structural integrity of the bone structure andthus requires surgical stabilization and filling of the defects withbiological materials such as bone grafts or cements.

One approach to treating many bone defects comprises injecting, packing,or filling the defect with biocompatible bone cement. Such bone cementsare generally formulations of non-resorbable biocompatible polymers suchas PMMA (polymethylmethacrylate), or resorbable calcium phosphate orcalcium sulphate cements, which allow for the gradual replacement of thecement with living bone. Both types of bone cements have been usedsuccessfully in the treatment of bone defects secondary to compressionfractures of the distal radius, the calcaneous, the tibial plateau, andthe vertebral body.

Historically, however, most applications of bone cements have beenlimited to open procedures in which the surgeon injects, packs, or tampsthe biological material under direct visualization of the defectmargins. Although direct visualization maximally allows the surgeon toidentify adjacent structures that may be compromised by the inadvertentplacement or injection of cement, less invasive means (apparatus andtechniques) to assist the surgeon in safely and effectively placingbiocompatible cements are generally desirable.

For example, one debilitating condition for which less invasive means totreat with injectable cement would be desirable is osteoporoticcompression fracture of the spine. More than 700,000 osteoporoticcompression fractures of the vertebrae occur each year in the UnitedStates—primarily in the elderly female population. Until recently,treatment of such fractures was limited to conservative, non-operativetherapies such as bed rest, bracing, and medications.

A relatively new procedure known as “vertebroplasty” was developed inthe mid 1980's to address the inadequacy of conservative treatment forvertebral body fracture. This procedure involves injecting radio-opaquebone cement directly into the fracture void through a minimally invasivecannula or needle under fluoroscopic control. The cement is pressurizedby a syringe or similar plunger mechanism, thus causing the cement tofill the void and penetrate the interstices of broken trabecular bone.Once cured, the cement stabilizes the fracture and reduces pain—usuallydramatically and immediately.

One issue associated with vertebroplasty is containment of the cementwithin the margins of the defect. For instance, an osteoporoticcompression fracture of the vertebral body may progress to an unstableintravertebral defect that is devoid of a cortical bone margin tocontain the cement, and such a defect becomes an abnormal psuedo-jointthat must be stabilized to progress to healing. Although the bestalternative for treating such an intravertebral defect is the directinjection of bone cement into the defect to stabilize the vertebralbody, there is a risk of cement flowing beyond the confines of the boneinto the body cavity.

Yet another significant risk associated with vertebroplasty is theinjection of cement directly into the venous system, since the veinswithin the vertebral body are larger than the tip of the needle used toinject the cement. A combination of injection pressure and inherentvascular pressure may cause unintended uptake of cement into thepulmonary vessel system, with potentially disastrous consequencesincluding embolism to the lungs.

One technique which has gained popularity in recent years is a modifiedvertebroplasty technique in which a “balloon tamp” in inserted into thevertebral body via a cannula approach to expand or distract thefractured bone and create a void within the cancellous structure. Knowntamps are inflated using pressurized fluid such as saline solution. Thetamping effect, which may compact the cancellous vertebral bone to theextent it forms a barrier layer, is caused by the inflation of a balloonmembrane that expands, thereby producing a radial force. When deflatedand removed, the membrane leaves a void that is subsequently filled withbone cement. Creating a void within the cancellous bone by compactingthe cancellous bone prior to injecting cement facilitates the use oflarger filling cannulas and more viscous cement, which has beendesirable because more viscous cement is less prone to unwanted orexcessive cement flow.

There are, however, a number of limitations associated with such balloontamp procedures. In particular, the balloon tamps currently known andused in the art may not produce sufficient forces to cause distraction.Partial healing of a chronic vertebral compression fracture placessignificant counterforce on the expanding membrane or container andlimits the ability of the membrane or container to achieve fullvertebral distraction, even while the patient is lying on the surgicaltable and the vertebral body is unloaded. Furthermore, as membranes areinflated with increasing pressure, radial forces are distributed equallyand indiscriminately to all bone surfaces in contact with the membrane.The membrane then preferentially expands within the bone in a directionoffering the least counterforce. In the vertebral body, this directionis lateral in the transverse plane. Since it is generally desirable tocorrect deformity in the saggital plane, distractive forces delivered byconventional expanding membranes may often prove to be ineffective. As aresult, a large void is created which destroys most of the remainingintact trabecular bone and which requires a large volume of cement forcomplete fill. Since toxicity and clinical complication rates increasewith increasing volume of cement injected, this large volume may havedeleterious clinical effects or may limit the extent of treatment toadjacent fractured levels.

Moreover, the long-term rate of success for treatment by injection ofbone cement or other filler material can be increased by interdigitationof the cement or other filler material with the surrounding cancelloustissue, since interdigitation prevents relative movement of fracturedbone fragments and thus relieves pain. Balloon tamps, however, are knownto compact cancellous tissue—even to the point of forming what has beenreferred to as a “barrier layer”—which unfortunately retards suchbeneficial interdigitation. Poor cement/bone interface strength has leadto post operative dislodgement or loosening of the cement bolus,requiring medical and surgical intervention.

According to another recent method for treating bone defects such asvertebral fractures, a flexible mesh bag or container—which by itself,i.e., prior to filling, is non-load-bearing—is inserted into the voidthat has been formed in the bone and filled with cement, bone chips, orother filler material. Upon filling, expansion of the bag can also causeundesirable compaction of the surrounding cancellous bone. Moreover,depending on the porosity or permeability (or lack thereof) of the bagor container, the bag or container may, by itself, partially orcompletely preclude any interdigitation of the cement or filler materialwith the surrounding cancellous tissue.

Therefore, although the development of vertebroplasty, balloon-tamping,and container-based treatments represented an advance over prior, directvisualization techniques for treating bone defects, there remains a needfor better means to repair and stabilize unstable intravertebral bodydefects (particularly those that have advanced to cortical wall defects)and other bone defects.

SUMMARY OF THE INVENTION

The present invention features a collapsible and self-restoringstent-type device used with bone cement or similar filler material(referred to generically herein as bone cement) to treat bone defects,particularly within the vertebral body. The device includes acollapsible, self-restoring wire lattice or braided primary structureand flow-directing features.

The primary structure serves to maintain patency of the cavity in whichthe device is inserted while the bone cement is being injected; it is inthat sense that the device is like a stent. (In this regard, a stent orstent-type device may be defined as a structure having sufficientstrength in radial compression to maintain the separation of two or moretissue surfaces surrounding a void within a bone defect or fracture,e.g., to maintain manually or otherwise generated separation ordistraction of the vertebral endplates during bone cement-basedtreatment of vertebral fractures.) However, unlike balloons, bags, orother container-type devices that have been used previously to generateand/or maintain separation or distraction, the primary structure of theinvention does so without compacting cancellous bone or forming abarrier layer around the cavity. Thus, the cancellous bone at themargins of the bone cavity has relatively normal trabeculararchitecture, and that fosters beneficial interdigitation of bone cementwith the surrounding cancellous bone.

The flow-directing features of the device, on the other hand, controlthe direction and rate of bone cement flow when cement is injected intothe cavity so as to avoid unwanted cement flow beyond the cortical bonemargins or into vascular sinuses or neural structures, all of which cancause clinical complications. Various flow-directing features arecontemplated, including (but not limited to) baffles attached to theprimary structure, holes or slots selectively formed in the primarystructure, and differential porosity of various regions of the primarystructure. Possible baffling elements may include co-braided filamentswhich occupy the spaces of the metal lattice or braid of the primarystructure without altering the formability or elasticity of the overallstructure, or they may include non-woven secondary films or coatingswhich adhere to regions of the lattice without altering the formabilityor elasticity of the structure. Overall, however, the device createsminimal total flow resistance or backpressure and thus directs, ratherthan contains, cement.

BRIEF DESCRIPTION OF THE FIGURES

These and other features and advantages of the invention will becomeclearer from the description below and the figures, in which:

FIGS. 1-4 are a perspective view, a top view, a side elevation view, andan end view, respectively, of one embodiment of a cement-directingdevice according to the invention;

FIGS. 5-8 are detail views illustrating possible braid configurationsused in the device shown in FIGS. 1-4;

FIGS. 9-11 illustrate intermediate steps in the construction of a deviceas shown in FIGS. 1-4;

FIGS. 12-16 are sequential side views illustrating a device according tothe invention loaded in and then being ejected from a cannula forinsertion into a bone cavity;

FIGS. 17-20 are views in the transverse plane and in the saggital planeof the vertebral body, illustrating the flow of cement into and througha device according to the invention and the resultant location ofhardened cement masses obtained thereby;

FIGS. 21 and 22 are an end view and a side elevation view of analternate embodiment of a cement-directing structure according to theinvention;

FIG. 23 is a sequence of side elevation views illustrating themanufacture of an alternate embodiment of a cement-directing deviceaccording to the invention;

FIG. 24 is a side view of another alternate embodiment of acement-directing device according to the invention;

FIGS. 25 and 26 are views in the transverse plane and in the saggitalplane in the vertebral body illustrating an alternate embodiment of acement-directing device according to the invention; and

FIG. 27 is a view in the saggital plane of a pair of vertebral bodiesillustrating an alternate embodiment of a cement-directing deviceaccording to the invention being used for spinal fusion followingdiscectomy.

DETAILED DESCRIPTION

A first embodiment 100 of a cement-directing structure according to theinvention is illustrated in FIGS. 1-11. As illustrated, the hollowstructure 100 may be generally ovoid or football-shaped in form(although various other shapes are also contemplated as falling withinthe scope of the invention, as noted below). In general, the structure100 features an elastic, self-restoring core member 102 that is crimped,welded, glued, sewn shut, or otherwise closed on one end 104 and open ornon-crimped at the opposite end 106; one or more cement flow windows 108(the illustrated embodiment having a pair of cement flow windows 108);and a flow-retarding baffle member 110. As will be explained in greaterdetail below, the cement flow windows 108 and the baffle member 110provide regions of differential cement flow permeability as compared tothe surrounding regions of the core member 102, and that differentialpermeability enables the structure 100 according to the invention tocontrol the direction in which cement flows when being injected into acavity formed within a bone. Although the embodiment 100 utilizes bothcement flow windows 108 and a baffle member 110, it is contemplated thatcement-directing structures according to the invention will worksatisfactorily if either only cement flow window(s) or only bafflemember(s) is or are used.

The core member 102 is formed from a multiplicity of elastic,heat-setting monofilament wire members (e.g., Nitinol wires) that arebraided together in a plain braid fashion to form a collapsible,self-expanding, generally tubular structure using techniques that areknown in the art. Other metallic or polymeric monofilament wires mayalso be used. The shape-memory/shape-restoring properties of alloys(particularly Nitonol), however, make them preferred. The core member102 has sufficient mechanical strength and elasticity to assume itsnominal shape upon complete insertion into a bone cavity and to contactopposed fractured surfaces, thereby providing some support to thesurfaces and maintaining patency of the cavity.

As illustrated in FIGS. 5-8, the preferred embodiment is actually aco-braided structure having primary wire members 112 with secondarymembers interwoven therewith. The secondary members may be smaller andmore elastic than the primary wire members 112, which are the membersthat are the primary source of the structure 100's shape andself-restoring capability. By way of non-limiting example, asillustrated in FIG. 5, the secondary members 114 may be monofilamentmetal wires that are finer than the primary wire members 112; asillustrated in FIG. 6, the secondary members 116 may be frayed polymericmultifilament yarns; as illustrated in FIG. 7, the secondary members 118may be flat wire or flat braid; or as illustrated in FIG. 8, thesecondary members 120 may be stranded multifilament wire. The type andproperties of the secondary members (e.g., number of wires, braidangles, diameters, etc.) will be selected to achieve a desired overallcombination of properties such as stiffness, strength, mesh density,etc.

The cement flow windows 108 are regions of the core structure 102 wherethe secondary members have been removed from the meshwork formed by theprimary wire members 112, thus leaving areas of increased permeabilityto cement flow relative to the surrounding regions of the core structure102. The rest of the surface of the structure, however, remainsco-braided. Thus, cement will tend to flow preferentially out of thecement flow windows 108 when it is injected into the interior of thecement-directing structure 100. The secondary members may be removedfrom the structure of the core member 102 by laser or mechanical cuttingafter the core member has been formed and heat-set in its desiredconfiguration.

The specific location of the cement flow window(s) 108 will, of course,depend on clinical intent. According to a presently preferredconfiguration, however, two cement flow windows 108 are provided.Lengthwise speaking, as best illustrated in FIG. 1 (only one cement flowwindow 108 being visible therein), the cement flow windows 108 areapproximately centered between the two ends 104 and 106 of thecement-directing structure 100, and the length of each window 108 isbetween approximately 50% and approximately 75% of the overall length ofthe structures. Circumferentially speaking, as best illustrated in FIG.4, each cement flow window 108 subtends approximately 30.degree. of arc,and the two cement flow windows 108 are located approximately120.degree. to approximately 160.degree. apart from each other (centerto center), symmetrically located above and below the lateral midplane109 of the structure 102.

The baffle 110, on the other hand, provides a region or regions ofdecreased permeability to cement flow as compared to the surroundingregions of the core structure 102. In other words, the baffle 110 blocksor severely restricts the flow of cement out of cement-directingstructure 102 in specific locations when cement is injected into theinterior of the structure 102. In this regard, the baffle 110 may beformed as an impermeable, flexible polymeric sheet or coating that isattached or bonded to either the inside or the outside of the corebraided structure 102. The flexible polymeric coating may be silicone orother biocompatible materials such as EPTFE (expandedpolytetroflouroethylene) or polyurethane, or it may comprise a tightlywoven fabric such as polyester or other biocompatible or degradablesuture material. The coating may be attached or adhered to the structureby a number of manufacturing processes known in the art, such as dipcoating or electrospinning. The approximate thickness of the bafflecoating is 0.0005 to 0.003 inches, so the coating will not precludeelastic deformation of the overall braided structure.

As is the case with respect to the cement window(s) 108, the preciselocation of the baffle 110 will depend on clinical intent. According tothe presently preferred embodiment, however, the baffle 110 extends allthe way from one end 104 of the device to the opposite end 106 andcovers the ends, as illustrated in FIGS. 1-3. Circumferentiallyspeaking, the baffle 110 subtends an arc of approximately 60.degree. toapproximately 80.degree., centered on and extending above and below thelateral midplane 109 of the structure 102.

Basic construction of a structure 100 according to the invention isillustrated in FIGS. 9-11. First, a hollow tubular structure like thatshown in FIG. 9 is formed on a braiding machine, as is known in the art.The structure will include the primary wire members 112 and anysecondary members (not shown in FIG. 9). The wires are then annealedsufficiently to maintain a specific diameter and to prevent the wiresfrom unraveling when the braided tube is cut to a specific length. Asillustrated in FIG. 10, the cut, braided tube is then placed over amandrel (shown in phantom) having the desired shape of the final productand collapsed down onto the mandrel, and further heat set into the finaldesired shape as illustrated in FIG. 11 (secondary members not shown).The mandrel is then removed by opening the wires on one end of thestructure, at which end the ends of the wire are left free or ungatheredin order to facilitate collapsibility of the structure for insertioninto a delivery catheter. The other end of the structure (e.g., the end104 in FIGS. 1-3) is then gathered and preferably crimped with a metaltube or sewn shut. If a metal crimp is used, it may be made fromradioopaque material (e.g. high-density metal such as platinum ortantalum) to facilitate location of the structure 100 within thevertebral body by means of fluoroscopy. (In addition to the crimp tube,at least some of the secondary members of the braid may also be madefrom radioopaque material such as platinum, or some of the wires of thestructure 102 (either primary or secondary) may be coated withradioopaque ink as known in the art.) The cement flow windows 108 arethen formed by selectively removing secondary members as describedabove, and the baffle 110 is formed, e.g., by coating the primary orcore structure 102 as also described above.

Insertion and cement-directing operation of a structure 100 within avertebral body VB is illustrated in FIGS. 12-20. In particular, thedelivery device used to insert the structure 100 is illustrated in FIGS.12-16, and the structure is illustrated in place and directing the flowof cement within the vertebral body in FIGS. 17-20.

As illustrated in FIGS. 12-16, since the structure 100 has one closedend 104 and one open end 106, it is preferred to collapse the structure102 over a hollow push rod 130 prior to inserting the structure 100 intoa catheter sheath 132, so that the push rod is effectively linked to theclosed end 104 of the structure 102. (The catheter sheath may, itself,include radioopaque marker bands 135, as is known in the art.) Thehollow push rod 130 facilitates placement of the collapsed, enclosedstructure into the cavity, via a cannula 133, through its removableconnection to the closed end of the structure. The removable connectionmay be a mechanical linkage, such as a thread or luer lock, or othersuitable attachment.

Once the sheath-covered structure 102 is fully inserted into the cavityformed within the bone structure being treated, the sheath 132 isretracted, as illustrated in FIG. 15 and the self-restoring structureexpands to its final shape, as illustrated in FIGS. 15 and 16. Theremovable connection between the closed end 104 of the structure 102 andthe push rod 130 is severed, and the hollow push rod 132 is partiallyretracted until its tip is located generally in the center of thestructure 102, at which point the push rod may be used secondarily as acement injector.

A filling portal (not shown) on the other end of the rod is thenconnected to a cement injection syringe via a luer lock fitting (notshown). Cement can then be injected into the center of the structure, asindicated by directional arrows shown in the cannula 134. It ispreferable that the open end of the structure be collapsed around thehollow push rod 132, thereby forming a slideable connection that assureslengthwise positioning and targeting of the flow portal of the push rod132 within the center axis of the self-restoring device and easy removalof the push rod after filling with cement.

(Alternatively, a separate filling needle (shown in phantom in FIG. 17)capable of penetrating the structure 102 could perforate the meshwork orbaffle of the structure after deployment of the structure in the bonecavity, so that the cement injection is not restricted to any particularvector; indeed, the structure 102 may be filled in multiple orientationsat multiple points of entry. By perforating the outer mesh, the needleflow portal may be placed in the center of the device or, if necessary,entirely through the device to regions of the bone external to thedevice where it may be desirable to inject cement directly into a bonefracture site.)

As illustrated in FIGS. 17-20, the regions of differentialpermeability—viz. the cement flow windows 108 and the baffle110—effectively control the direction of flow of cement into thevertebral body into which the structure 100 is inserted.

In particular, a greater amount of cement will flow out of the cementflow windows 108, as represented by the relatively thick, large arrows,than will flow out of the remainder of the structure 100, as representedby the relatively thin, small arrows. For the given orientation of thestructure 100 within the vertebral body, with the cement flow windows108 facing anterior-superior and anterior-inferior and the baffle 100facing posterior, significant masses M of cement will be directedanterior-superior and anterior-inferior into the forward third of thevertebral body, thereby forming “mantles” of cement which cross theplane of the vertebral fracture. The cement “mantles” will be locatedadjacent to the vertebral endplates and thus will form a load-bearingcolumn of cement.

Where other flow of cement out of the structure 100 exists, smallervolumes or masses m of cement will form. These smaller masses m ofcement will beneficially interdigitate with the surrounding healthy bonetissue, thereby helping to anchor the structure of the invention inplace within the vertebral body VB.

Conversely, the baffle 110, which is impermeable to cement, will blockthe flow of cement out of the structure 100 in the posterior direction.Advantageously, this helps prevent cement from flowing posteriorly,e.g., into the posterior venous complex, spinal canal, etc.

A modified embodiment 200 of a cement-directing structure according tothe invention is illustrated in FIGS. 21 and 22. In this embodiment, thebaffle 210 further includes a flexible, sheet-like material 211 formedfrom either solid polymer film, non-woven polymer film, or woven fabricthat is connected to the primary braided structure 202 along a singleregion 203 and that extends from the primary braided structure so thatthe flexible sheet-like material 211 may be wrapped around the structure202 without being adhered to the structure. The sheet-like structure 211is therefore unconstrained and may open independently of theself-restoring structure 202. This embodiment 200 allows cement to flowfreely through the structure 202 and contact the sheet-like bafflestructure 211. The force of cement contact will cause the sheet-likebaffle 211 to open to the limits defined by the bone cavity, or until anequilibrium between forces of flowing cement with the resistive force ofthe bone cavity is attained.

Other variations in the invention are also possible. For example, usinga variant of the manufacturing method described above, a multi-layeredbraided structure can be formed. For example, it is known that braidscan be formed in multiple layers over a mandrel. Alternatively, theoriginal braided tube structure may be folded back on itself prior toheat-setting, as illustrated in FIG. 23, to create a double-layered ormultiple-layered braided structure. Double- or multi-layering of thebraided structure increases stiffness of the structure and can assist indirecting the flow of cement by increasing the mesh density where layersof the structure overlap.

Another multi-layered variation 300 of a cement-directing structureaccording to the invention is shown in FIG. 24, wherein two additionallayers 302, 304 of elastic braided filament are nested within theoutermost layer 306. These sequential layers could be formed over amandrel (not shown) then heat-set together. The multiple layers may allbe collapsed into a tubular form (not shown) while nested togetherbefore being slidingly fit into a sheath.

If the stacked, multi-layer structure is too thick to fit in a sheath inthe collapsed state for deployment, then a layered structure may beconstructed in vivo by deploying individual self-restoring structuressequentially into the original expanded structure. In that specificinstance, the outer layer 306 would have an opening sufficient to acceptthe second layer 304 such that when assembled in vivo the second layeroccupies the opening of the first layer. The inner expandable structures302, 304 may or may not have a baffling component, supplementaryfilaments, or coatings, yet would provide enhanced mechanical strengthas each layer expands and contacts the outer layer. Each consecutivedevice would be pre-assembled in the collapsed state into a cannula andthen deployed through the cannula in sequence (not shown). The pluralityof layers defined in this alternate embodiment has the secondary benefitof reinforcement to the cement mantle.

In addition to these variant embodiments, shapes other than the ovoid orfootball shape shown in the Figures above may be desirable. For example,oblong or pear-shaped cement-directing structures 400 might be desiredwhere, for example, when it is clinically indicated to approach thevertebral body bilaterally, through each pedicle, as illustrated inFIGS. 25 and 26. Alternatively, a relatively thin cement-directingstructure 500 might be employed for spinal fusion techniques, asillustrated in FIG. 27, wherein the cement-directing structure isdeployed through minimally invasive means into the disc space to providesupport to the spinal column following discectomy. Finally, although amesh structure formed by braiding has been disclosed and describedabove, those having skill in the art will appreciate thatself-expanding, collapsible mesh structures can be formed by a varietyof other techniques, e.g., laser-cutting tubes, etc, and the inventionis not limited to braided mesh structures.

These and other variations to the embodiments disclosed and describedabove will occur to those having skill in the art. To the extent suchvariations incorporate the inventive concepts disclosed herein, they aredeemed to fall within the scope of the following claims.

I claim:
 1. A device for use in treating orthopedic defects in aprocedure comprising inserting the device into a pre-formed cavitywithin a bony member and injecting bone cement or other orthopedicfiller material into an interior region of the device, said devicecomprising: a collapsible, self-expanding mesh structure having a first,collapsed configuration and a second, expanded configuration, saidfirst, collapsed configuration enabling the device to be inserted intothe pre-formed cavity through a cannula and said second, expandedconfiguration being sufficient to maintain walls of the cavity apartfrom each other, wherein said mesh structure itself has regions ofdifferential permeability or flow resistance, and wherein said meshstructure is a co-braided structure comprising primary members andsecondary members occupying interstices between said primary members,and wherein said mesh structure has regions of increased permeability ordecreased flow resistance in which secondary members are not present. 2.The device of claim 1, wherein, when in said second configuration, thedevice does not compress cancellous bone when the device is positionedwithin the cavity and the cavity is formed within cancellous bone. 3.The device of claim 1, wherein said mesh structure has at least oneregion in which the mesh opening size of said region is greater than themesh opening size of surrounding regions, whereby bone cement or otherorthopedic filler material flows preferentially out of the devicethrough said at least one region of greater mesh opening size.
 4. Thedevice of claim 1, further comprising at least one flow-retarding bafflemember.
 5. The device of claim 4, wherein said flow-retarding bafflecomprises a coating formed on select regions of said mesh structure. 6.The device of claim 4, wherein said flow-retarding baffle comprises asheet-form material that is attached to select regions of said meshstructure.
 7. The device of claim 4, wherein said flow-retarding bafflemember comprises one or more wing-like members extending from said meshstructure.
 8. The device of claim 1, wherein said mesh structure itselfhas regions of differential permeability or flow resistance and whereinsaid device further comprises at least one flow-retarding baffle member.9. The device of claim 1, wherein said device is generally hollow andhas first and second ends, both of which are closed.
 10. The device ofclaim 9, wherein said first ends is fastened closed and said second endis unrestrained.
 11. The device of claim 1, wherein said devicecomprises a stent.
 12. A cement-directing device, comprising: acompressible, shape-restoring hollow mesh structure having a compressedshape and an expended shape, wherein the compressed shape is capable ofbeing inserted into and moved through a cannula, the expanded shapebeing approximately football-shaped, and the mesh structure havingsufficiently open mesh area to create minimal added flow resistance tobone cement inserted into the device, wherein said mesh structure itselfhas regions of differential permeability or flow resistance, and whereinsaid mesh structure is a co-braided structure comprising primary membersand secondary members occupying interstices between said primarymembers, and at least one baffle member attached to the hollow meshstructure and configured to direct the flow of bone cement in preferreddirections and to impede the flow of bone cement in non-preferreddirections.
 13. The device of claim 12 wherein said device comprises astent.
 14. A cement-directing device suitable for use in repairing abone defect, said device comprising a generally tubular, collapsible andself-expanding mesh framework, said device having regions ofdifferential permeability or resistance to the flow of bone cementtherethrough; wherein said mesh framework has interstices that are sizedto permit a bone cement injection needle to pass through said meshframework to inject bone cement into an interior cavity of said device;and wherein said device is generally hollow and has first and secondends, both of which are closed; and said first end is fastened closedand said second end is unrestrained.
 15. The device of claim 14, furthercomprising one or more baffling members attached to said mesh framework,said baffling members providing regions of decreased permeability to theflow of bone cement.
 16. The device of claim 14, wherein said regions ofdifferential permeability or resistance to the flow of bone cementtherethrough are provided by regions of differential mesh density. 17.The device of claim 16, wherein said mesh framework is a multi-layerstructure.
 18. The device of claim 12 wherein said device comprises astent.